Automotive electrical systems provide a number of operating, safety and convenience functions using various control modules. These systems and control modules ultimately derive their electrical power from the vehicle battery. When the engine is not operating, the battery supplies power without simultaneously recharging thereby draining the battery making it ineffective to start the vehicle engine when required to do so. Many electronic modules have an inactive or sleep mode that reduces power consumption when the ignition is off.
FIG. 1 is an example of using a conventional wake up circuit to conserve battery energy by waking up a microprocessor controller from a sleep mode when a demand is made to power up a system to achieve a particular function. The control module (climate control, cluster, radio, etc.) is powered by the battery. Full operation is provided when ignition is detected by an ignition detector. When the signal from the ignition detector is removed, the module goes into a sleep mode where it draws a minimal amount of energy from the battery. While in the sleep mode, the microprocessor and communications module monitor for messages from other vehicle modules. One message that must be processed is a command to light the front panel of the dashboard display if the headlights are turned on when the ignition is off. Once the command is received the microprocessor exits the sleep mode and powers the display and bezel lighting to the proper level via a battery switch using a voltage regulator.
Due to the requirement to keep the battery energy drain as low as possible, the voltage regulator is costly in terms of dollar cost and energy consumption during the sleep mode. To minimize battery drain a low quiescent current voltage regulator is required which is expensive in terms of dollars compared to a standard voltage regulator. The microprocessor design is complicated by the need to have a sleep mode, and an additional pin is required to allow the battery power to be switched to the display/lighting circuit. In spite of an expensive voltage regulator and microprocessor, a finite amount of energy is required to allow the voltage regulator, microprocessor and communications module to operate to sense the incoming messages. This energy drain adds to the cost of the battery and the rest of the charging system requirements. Accordingly, it is desirable to power up when a demand for energy is made but to use minimal energy when the engine is off until a demand is made.